Vapochromic photodiode

ABSTRACT

A sandwich-type, organic p-n junction photodiode is prepared by electrode-position of an insoluble layer of oligomerized tris(4-(2-thienyl)phenyl)amine onto conducting indium-tin oxide coated glass, spin-coating the stacked platinum compound, bis(cyanide)-bis(para-dodecylphenylisocyanide)platinum (II), from chloroform onto the oligomer layer, and then coating the platinum complex with a micro-array of aluminum electrodes by vapor deposition. This device shows rectification of current and gives a measurable photocurrent. The photocurrent action spectrum follows the absorption spectrum of the platinum complex; changes in the action spectrum with layer thickness point to a p-n junction formed at the interface of the molecular layers as the site of rectification. Exposure of the device to acetone vapor causes the action spectrum to shift dramatically to longer wavelength. Exposure to chloroform vapor causes a return to the original spectrum. These results demonstrate a new type of photosensor that reports the arrival of organic vapors.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part of application Ser.No, 09/225,758, filed Jan. 5, 1999, entitled “A Vapochromic LED” and adivisional of application Ser. No. 09/315,877, filed May 20, 1999, nowU.S. Pat. No. 6,137,118. That application is directed to molecular lightemitting diodes based on thin films of certain platinum complexes.

The Government has certain rights in this invention, pursuant to grants(NSF/CHE/9216003 and NSF/CHE/9307837) from the National ScienceFoundation.

TECHNICAL FIELD

The present invention relates generally to molecular electronics, and,more particularly, to vapor-sensitive, molecular photodiodes based onthin films of certain platinum complexes.

BACKGROUND ART

The inventors have recently published reports that have enucleated andexplained the unusual “vapochromic” changes in absorption and emissionspectra that result when certain stacked platinum complexes are exposedto organic vapors; see, e.g., C. L. Exstrom et al, Chemical Materials,Vol. 7, pp. 15-17 (1995) and C. A. Daws et al, Chemical Materials, Vol.9, pp. 363-368 (1997).

A typical experiment involves a solution, crystal or solid film ofmaterial, such as tetrakis(p-decylphenylisocyano)platinumtetracyanoplatinate (I) (see FIG. 1, which depicts the chemical formulaof the compound, where the dashed vertical line indicates the c-axis)that forms stacks of alternating cations and anions with stronginterplatinum interactions. These salts exhibit an intense absorptionband in the visible region. Exposing the stacks to small moleculevapors, such as acetone or chloroform, leads to sorption of the vapormolecules in the free volume between the stacks, and produces shifts inthe absorption and emission spectra. These “vapochromic” or“vapoluminescent” changes are usually reversible so that the originalspectrum is regained quickly after the vapor is removed. Such an effecthas potential application for sensor technology.

The inventors developed a new type of sensor technology, called the“vapochromic LED”; see, Y. Kunugi et al, Journal of the AmericanChemical Society, Vol. 120, pp. 589-590 (January 1998) and applicationSer. No. 09/225,758, listed above.

A sandwich LED was prepared using compound 1 that gaveelectroluminescence from this platinum compound; see, e.g., R. H. Friendet al in Physical Properties of Polymers Handbook, J. E. Mark, Ed., AIPPress (1996); Y. Yang, MRS Bulletin, pp. 31-38 (June 1997); T. Tsutsui,MRS Bulletin, pp. 39-45 (June 1997); W. K Salaneck et al, MRS Bulletinpp. 46-51 (June 1997); and C. Hosokawa et al, Synth. Met., Vol. 91, pp.3-7 (December 1997). Exposure of the device to an organic vapor sharplychanged the wavelength of electroluminescence, thereby providing a newmethod for remote vapochromic sensing which does not require a lightsource.

On the other hand, photodiodes, which do require a light source, havefound extensive use in the electronics industry. Organic and polymerphotodiodes have been built using materials such aspoly(3-alkylthiophene)s, oligothiophenes, and C₆₀. These photodiodesgive photocurrents corresponding to the absorption of light by themolecular materials. They are of interest because they can givewavelength selectivity and quantum efficiencies of more than 10%electron/photon under modest reverse bias; see, e.g., H. Yonehara et al,Applied Physics Letters, Vol. 61, pp. 575-576 (August 1992); G. Yu etal, Applied Physics Letters, Vol. 64, pp. 422-3424 (June 1994); and Y.Kunugi et al, Chemical Materials, Vol. 9, pp. 1061-1062 (May 1997).

However, to the best of the present inventors' knowledge, a photodiodewhich can detect the arrival of organic vapors has not been described.Such a device would be of interest in detecting the presence of organicvapors by a change in photocurrent.

DISCLOSURE OF INVENTION

In accordance with the present invention, a molecular photodiode isprovided. The molecular photodiode employs an organic complex that actsas both a sensor to certain organic molecules, or analyte vapors, and asan active light emitter. The molecular photodiode of the presentinvention comprises:

(a) a first electrode;

(b) a first molecular layer formed on the first electrode, capable of atleast transporting charge;

(c) a sensing/emitting layer formed on the first electrode, thesensing/emitting layer comprising a material that changes color uponexposure to the analyte vapors and that forms a rectifying junction withthe first molecular layer; and

(d) a second electrode formed on the sensing/emitting layer, wherein atleast the first electrode comprises an optically transparent material.The device is preferably formed on a transparent dielectric substrate,on which the first electrode is formed.

Also in accordance with the present invention, a method is provided fordetecting analyte vapors. The method comprises:

(a) providing the above-described vapochromic photodiode;

(b) applying a voltage to the two electrodes in the range of 0 to about50 V;

(c) shining light on the device in the wavelength range of about 300 to1,000 nm;

(d) introducing the analyte vapors to the sensing layer; and

(e) measuring the photocurrent prior to and subsequent to exposure ofthe device to the analyte vapors to obtain a change in the photocurrent.

Further in accordance with the present invention, methods are providedfor forming the vapochromic photodiode.

Other objects, features, and advantages of the present invention willbecome apparent upon consideration of the following detailed descriptionand accompanying drawings, in which like reference designationsrepresent like features throughout the FIGURES. The drawings referred toin this description should be understood as not being drawn to scaleexcept if specifically noted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1 b depict the basic formulae of certain platinum complexes,specifically, tetrakis(p-decylphenylisocyano)platinumtetracyanoplatinate (Compound 1) andbis(cyanide)-bis(p-dodecylphenylisocyanide) platinum (II) (Compound 2)employed in the practice of the present invention;

FIGS. 1c-1 d depict film-forming, light-emitting compounds (notvapochromic) that are useful in the practice of the invention, in whichCompound 3 is an oligomer of Compound 4 (tris(-2-thienyl)phenyl)amine);

FIG. 2, on coordinates of intensity (in arbitrary units) and wavelength(in nm), is a plot of the absorption spectra of (a) Compound 2 beforeexposure to acetone vapor, (b) Compound 2 after exposure to acetonevapor, and (c) Compound 3;

FIG. 3a is a side elevational view of the molecular photodiode of thepresent invention, using two separate layers, one forbis(cyanide)-bis(p-dodecylphenylisocyanide) platinum (II) (Compound 2)and one for electrically oligomerized tris(4-(2-thienyl)phenyl)amine(Compound 3);

FIG. 3b is a top plan view of the molecular photodiode depicted in FIG.3a;

FIG. 4, on coordinates of photocurrent (in arbitrary units) andwavelength (in nm), is a plot depicting the action spectra for theITO/Compound 3/Compound 2/Al photodiode cell of FIGS. 3a-3 b (a) beforeexposure to acetone and (b) after exposure to acetone; and

FIG. 5, on coordinates of photocurrent (in arbitrary units) andwavelength (in nm), is a plot depicting the time dependence of thephotocurrent for the ITO/3/2 photodiode cell of FIGS. 3a-3 b illuminatedwith 560 nm light.

BEST MODES FOR CARRYING OUT THE INVENTION

Reference is now made in detail to a specific embodiment of the presentinvention, which illustrates the best mode presently contemplated by theinventors for practicing the invention. Alternative embodiments are alsobriefly described as applicable.

Herein is described a vapochromic photodiode based on chemical andelectrical effects related to those involved in the LED disclosed andclaimed in the related application, Ser. No. 09/225,758.

To the knowledge of the present inventors, a photodiode which can detectthe arrival of organic vapors has not been heretofore described. Inaccordance with the present invention, such a photodiode comprises anelectrode transparent to the incident light, a second molecular layerthat forms a rectifying junction with an emitter layer, a third layer ofan appropriate vapochromic material that functions as the emitter layer,and a second electrode that permits vapor to reach the molecularmaterials.

Bis(cyanide)-bis(p-dodecylphenylisocyanide) platinum (II) (Compound 2,FIG. 1b) was chosen as the stacked platinum complex because it formsgood films by spin casting and it exhibits a large color change inresponse to organic vapors. This complex was disclosed by C. L. Exstrom,Ph.D. Dissertation, University of Minnesota (1995). Compound 2 wasprepared by a melt method, as disclosed by Exstrom, supraTetrakis(p-decylphenylisocyano)platinum tetracyanoplatinate, from C. A.Daws et al, supra, (1.785 g, 1.20 mmoles) was placed in a flask and leftunder an argon atmosphere. As the deep blue sample was slowly heated to165° C., the solid melted forming a reddish brown liquid. Upon cooling,the liquid solidified. The resulting solid was extracted from the flaskwith dichloromethane and purified by column chromatography with 17:3dichloromethane:ethyl acetate (1.526 g, 2.08 mmoles, 87% yield). Thepurified product gave appropriate combustion analysis and spectroscopicdata, as listed by Exstrom, supra.

The color of the film cast from chloroform solution was yellow (λ_(max):400 nm);

see Curve 10 in FIG. 2. Exposure of the film to acetone vapor changedthe color to reddish-purple (λ_(max): 560 nm); see Curve 12 in FIG. 2.This reddish-purple color was stable for several months under ambientlaboratory conditions, but could be switched back to yellow by exposureto chloroform or dichloromethane vapor. Devices were prepared by spincasting Compound 2 from chloroform solution onto electrically-conductingindium-tin oxide (ITO) coated glass, followed by vapor deposition ofaluminum. These devices, like single layer devices formed from Compound1 (FIG. 1a) and reported by Y. Kunugi et al, supra, were very resistive,unstable, and did not show rectification.

Recently, the inventors have shown that electrochemical oligomerizedtris(4-(2-thienyl)phenyl)amine (Compound 3, FIG. 1c) can be used as ahole transport layer to make a two-layer device; see, Y. Kunugi et al,supra. Because the layer of Compound 3 is insoluble in solvents such aschloroform or acetone, it was possible to spin cast a layer of Compound2 on top of Compound 3 without layer interdiffusion. The two-layerdevice ITO/3/2/Al was, therefore, prepared by anodically oxidizingtris(p-thienylphenyl)amine (Compound 4, FIG. 1d) in acetonitrile,lithium perchlorate providing the oxidized form of Compound 3, and thenelectrochemically reducing it to form the neutral Compound 3 (filmthickness=200 nm). The absorption spectrum of Compound 3 as a functionof wavelength is depicted in FIG. 2, Curve 14. After drying, a layer ofCompound 2 (film thickness=250 nm) was spin-cast from chloroform on topof Compound 3, and then aluminum (150 nm thick) was vapor depositedthrough a mask onto Compound 2. To insure a quick response to the vapor,the aluminum electrode was fabricated as fingers spaced apart by 0.1 mm(FIGS. 3a-3 b).

FIGS. 3a-3 b depict one embodiment of the vapochromic photodiode 16 ofthe present invention. The transparent indium tin oxide (ITO) layer 18is formed on a transparent substrate 20. A layer 22 of Compound 3 isformed on the ITO layer 18. A layer 24 of Compound 2 is formed on thelayer 22. An aluminum electrode 26 is formed on the top of the layer 24.

The transparent electrode 18 may comprise indium tin oxide orpolyaniline, while the second electrode 26 may comprise aluminum,magnesium, calcium, or silver.

The molecular layers 22 and 24 are typically less than 1,000 nm thick.

The device ITO/3/2/Al gave rectification (rectification ratio 100 at ±5V) of the current favoring electron flow from Al 26 through themolecular layers 24, 22 to ITO 18. For photocurrent action spectra, a450 W Xe lamp was used, and the photocurrent was normalized to accountfor the variation of lamp output with wavelength. A calibrated siliconphotodiode was used to measure light intensities. The photocurrentaction spectrum of the device ITO/3/2/Al has a broad peak between 400and 500 nm (Curve 28 of FIG. 4). Exposure to argon saturated withacetone vapor gave a shifted action spectrum (Curve 30 of FIG. 4)(λ_(max): 560 nm), corresponding to the shifted absorption spectrum ofCompound 2 (Curve 12 of FIG. 2). This spectrum was stable in air in theabsence of acetone vapor. Exposure of the device to chloroform vapor inargon (or air) caused the spectrum to revert to the original. The shapesof these spectra are independent of light intensity. Illuminated throughITO with 560 nm light (1.0 mW cm⁻²), the quantum efficiency(electron/photon of absorbed light) of the device 16 was 0.03% at 0 Vand 16.5% at 20 V reverse bias. It is noted that Compound 2 is visiblyphotoluminescent and so at low bias voltages, emission of light caneffectively compete with charge separation and photocurrent generation.

The known instability of polymer-based electro-optical devices duringoperation in air is not such a severe problem for sensors. As a sensor,the device 16 only needs to be pulsed occasionally to check for thearrival of the vapor of interest and so operating times can be quiteshort. In the present case, the photocurrent decayed 40% duringcontinuous operation in air. When pulsed, the response was much morestable and this allowed measurement of the time dependence of switching.The photocurrent for the ITO/3/2/Al cell 16 illuminated with 2 secpulses of 560 nm light is shown in FIG. 5. When acetone vapor m argonwas introduced above the device 16, the photocurrent increased more than10 times in 2 min. This response time is much faster than that forpreviously-reported vapochromic LED (Y. Kunugi et al, supra). This seemsto be a result of the use of the microelectrode array 26, instead ofrelying on the slower diffusion through a porous aluminum layer.

Of interest in terms of mechanism is the action spectrum between 400 and500 nm (FIG. 4), which does not correspond closely with the absorptionspectra of either Compound 2 or Compound 3 (FIG. 2). To understand thisdifference, a device with a thicker layer of Compound 3 was prepared.The efficiency of this device was smaller at all wavelengths, butespecially between 350 and 460 nm. The weaker action spectrum between500 and 700 nm still followed the absorption spectrum of Compound 2.These effects appear to result because the active interface is thatbetween Compound 2 and Compound 3, and because Compound 3 acts as anoptical filter; see, e.g., A. K. Ghosh et al, Journal of AppliedPhysics, Vol. 45, pp. 230-236 (January 1974) and Y. Harima et al,Applied Physics Letters, vol. 45, pp. 1144-1145 (November 1984). If theITO/3 interface is not active, then the thicker layer of compound 3 onlyacts to attenuate the light reaching the region near the active 3/2interface 32. A p-n junction is formed at the 3/2 interface 32 and theorganic layers, especially Compound 2, absorb light and give chargeseparation near this interface.

These results demonstrate a new class of gas sensors that report thearrival of organic vapors by a change in photocurrent. In one sense,these photodiodes are like vapochromic absorption sensors without therequirement for a separate detector of the absorbed light. Sincedifferent platinum complexes and/or organic vapors will elicit differentoptical responses, a variety of chemicals can be detected and there aremany opportunities for improving and fine tuning device performance.

INDUSTRIAL APPLICABILITY

The photodiodes of the present invention are expected to find use in thedetection of vapors.

Thus, there has been disclosed a vapochromic photodiode. It will bereadily apparent to those skilled in this art that various changes andmodifications of an obvious nature may be made, and all such changes andmodifications are considered to fall within the scope of the presentinvention, as defined by the appended claims.

What is claimed is:
 1. A method for detecting analyte vapors comprising:(a) providing a vapochromic photodiode comprising (1) a first electrode,(2) a first molecular layer formed on the first electrode, said firstmolecular layer capable of at least transporting charge, (3) asensing/emitting layer formed on the first electrode, thesensing/emitting layer comprising a material that changes color uponexposure to the analyte vapors and that forms a rectifying junction withthe first molecular layer, and (4) a second electrode formed on thesensing/emitting layer, wherein at least the first electrode comprisesan optically transparent material; (b) applying a voltage to said firstand second electrodes; (c) exposing said photodiode to light; (d)introducing said analyte vapors to said photodiode; and (e) measuringphotocurrent prior to and subsequent to exposure of said photodiode tosaid analyte vapors to obtain a change in said photocurrent.
 2. Themethod of claim 1 further including means for introducing said analytevapors to said sensing layer.
 3. The method of claim 1 wherein saidvapochromic photodiode further includes a substrate on which said diodeis formed, with said first electrode formed on said substrate.
 4. Themethod of claim 1 wherein said first electrode comprises an opticallytransparent material selected from the group consisting of indium tinoxide and polyaniline.
 5. The method of claim 1 wherein said secondelectrode comprises a metal selected from the group consisting ofaluminum, magnesium, calcium, and silver.
 6. The method of claim 1wherein said voltage is in a range of 0 to about 50 Volts.
 7. The methodof claim 1 wherein said light is in a wavelength range of about 300 to1,000 nm.
 8. The method of claim 1 wherein following step (c), a firstphotocurrent is measured and following step (d) a second photocurrent ismeasured, with said change in photocurrent comprising a differencebetween said first photo-current and said second photocurrent.